This invention relates generally to non-ferrous metal alloy compositions, and more specifically to nickel-chromium-molybdenum-copper alloys that provide a useful combination of resistance to sulfuric acid and resistance to xe2x80x9cwet processxe2x80x9d phosphoric acid.
One of the steps in the manufacture of fertilizers involves a reaction between phosphate rock and sulfuric acid, to create xe2x80x9cwet processxe2x80x9d phosphoric acid. In this reaction step, there is a need for materials resistant to both sulfuric acid and xe2x80x9cwet processxe2x80x9d phosphoric acid. Alloys currently considered for such applications include austenitic stainless steels and nickel-iron alloys containing high levels of chromium, in the approximate range 28 to 30 wt. %. Among these are G-30 alloy (U.S. Pat. No. 4,410,489), Alloy 31 (U.S. Pat. No. 4,876,065), and Alloy 28. Alloys with even higher combined resistance to these two acids are sought, however.
It is known that chromium is beneficial to the corrosion resistance of iron-nickel and nickel-iron alloys in xe2x80x9cwet processxe2x80x9d phosphoric acid. It is also known that copper benefits the resistance of these same alloy systems to sulfuric acid, and that molybdenum is generally beneficial to the corrosion resistance of nickel alloys. The use of these alloying additions, however, is constrained by thermal stability considerations. In other words, if the solubilities of these elements are exceeded by a significant amount, it is difficult to avoid the precipitation of deleterious intermetallic phases in the microstructure. These can influence the manufacturing of wrought products and can impair the properties of weldments.
Given that chromium, molybdenum and copper are more soluble in nickel than iron, it follows that higher levels of these elements are possible in low iron, nickel alloys. It is not surprising, therefore, that molybdenum-bearing nickel alloys with high chromium contents exist. U.S. Pat. No. 5,424,029 discloses such a series of alloys, although these require the addition of tungsten, in the range 1 to 4 wt. %, and do not require copper. U.S. Pat. No. 5,424,029 states that such alloys possess superior corrosion resistance to a variety of media, although they were neither tested in pure sulfuric acid nor xe2x80x9cwet processxe2x80x9d phosphoric acid. Notably, U.S. Pat. No. 5,424,029 states that the absence of tungsten results in a significantly higher corrosion rate. Also notably, it states that corrosion resistance worsens significantly when copper is present at levels of 1.5% or greater.
Another patent which discloses corrosion-resistant, molybdenum-bearing, nickel alloys with high chromium contents is U.S. Pat. No. 5,529,642, although the preferred chromium range is 17 to 22 wt. %, and all compositions require the addition of tantalum, in the range 1.1 to 8 wt. %. Copper is optional in the alloys of U.S. Pat. No. 5,529,642, up to 4 wt. %.
Two further U.S. Pat. Nos. 4,778,576 and 4,789,449, disclose nickel alloys with wide-ranging chromium (5 to 30 wt. %) and molybdenum (3 to 25 wt. %) contents, for use as anodes in electrochemical cells. Both patents preferably claim anodes made from C-276 alloy, which contains 16 wt. % chromium and 16 wt. % molybdenum, but no copper.
The principal object of this invention is to provide new, wroughtable alloys with higher combined resistance to sulfuric acid and xe2x80x9cwet processxe2x80x9d phosphoric acid than previous alloys. It has been found that the above object may be achieved by adding chromium, molybdenum, and copper to nickel, within certain preferred ranges, together with elements required for sulfur and oxygen control, during melting, and unavoidable impurities. Specifically, the preferred ranges in weight percent are 30.0 to 35.0 chromium, 5.0 to 7.6 molybdenum, and 1.6 to 2.9 copper. The most preferred ranges in weight percent are 32.3 to 35.0 chromium, 5.0 to 6.6 molybdenum, and 1.6 to 2.9 copper.
For control of sulfur and oxygen, during argon-oxygen decarburization, up to 1.0 wt. % manganese, and up to 0.4 wt. % aluminum are preferred. Most preferred for this purpose are 0.22 to 0.29 manganese and 0.20 to 0.32 aluminum. Silicon and carbon are also necessary ingredients during argon-oxygen decarburization, levels up to 0.6 wt. % and 0.06 wt. %, respectively, being preferred. Nitrogen and iron are non-essential, but desirable, minor additions. Nitrogen levels up to 0.13 wt. % are preferred; iron levels up to 5.1 wt. % are preferred. With regard to likely impurities, up to 0.6 wt. % tungsten can be tolerated. Up to 5 wt. % cobalt can be used in place of nickel. It is anticipated that small quantities of other impurities, such as niobium, vanadium, and titanium would have little or no effect on the general characteristics of these materials.